Datasheet

3 Design Procedure Divider Network
www.ti.com Design Procedure Divider Network
The TPS62110 feedback voltage (FB) is set to 1.153 V. If this is used for current regulation, the power
dissipation of the sense resistor is high. To reduce the power dissipation on the current sense resistor R
10
,
a reference voltage of 2.5 V is summed resulting in the reduction of the V
sense
voltage from 1.153 V to
0.255 V.
The first step is to choose the LED operating current and V
sense
voltage which will determine the value for
R
10
. A tradeoff occurs, however, between the power dissipation on R
10
and the accuracy of the regulation
point. As V
sense
is increased, the current regulation accuracy improves; the maximum voltage will be 1.153
V. As V
sense
is decreased, the power dissipation is reduced, but error due to V
ref
, resistors R
1
and R
2
increases.
The EVM I
LED
is set to 500 mA with a V
sense
voltage of 0.225 V.
V
sense
= I
LED
× R
10
= 500 mA × 510 m Ω = 0.255 V
V
R1
= V
FB
– V
sense
= 1.153 V – 0.255 V = 0.898 V
V
R2
= V
REF
– V
FB
= 2.5 V – 1.153 V = 1.347 V
Choose divider current of 1 mA:
Standard value 887:
Standard value 1330
Output current can be increased or decreased by changing R
10
. For 1-A output current, decrease R
10
to
250 m Ω . For 250-mA output current, increase R
10
to 1 Ω .
SLVU254 – September 2008 TPS62110EVM-346 5
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